A trend in personal portable devices (such as cell phones and personal digital assistants) is to add new features while keeping the devices small. Many of the new features, such as photograph sharing and video downloading, depend upon a high resolution, easy-to-read display screen. However, manufacturers cannot simply keep increasing the size of their display screens because that would eventually run counter to the desire to keep the devices small and portable.
Recently, “microprojectors,” a new category of display device, have been designed to address this conflict between greater display area and smaller device size. An image, either still or moving, is projected from the device onto a convenient surface (e.g., a projection screen or an office wall). The maximum size of the image is then effectively constrained by the amount of available wall space rather than by the size of the device itself. Using a microprojector-equipped device, several people can simultaneously view a photograph, for example, or review a full page of text, neither of which can be readily done with even the largest displays on current personal portable devices.
Promising as they are, microprojectors raise new headaches when engineers attempt to fit them into personal portable devices. While the overall size of the projected image may be effectively unlimited, expanding the image size is of little use if the resolution of the projected image is severely constrained. What customers want is a projected image that is both larger overall and has much greater resolution than a device's display screen. But, generally, the overall size of a microprojector grows with the amount of resolution it provides. This is especially true when a microprojector uses a microdisplay imager as its image source. The trend toward very thin personal portable devices renders it a challenge to fit in a microprojector that provides usefully high resolution.
Power use is another challenge. By its nature, a microprojector uses a significant amount of power to light a large display area. In addition, microprojectors usually use proven liquid-crystal displays which only work with linearly polarized light. Light created for use by the microprojector is first sent through a polarizer, a component that discards about half of the original light and thus discards about half of the power. Reducing the physical size of the microprojector exacerbates the power problem because the optics in microprojectors become less power-efficient as they become smaller. Designers of battery-based personal portable devices are already concerned about their power budgets and look askance at any new feature that threatens to reduce the utility of the device by reducing how long the device can operate between charges.